Fifty-six of the volunteers, of various ages and both sexes, turned out to be positive for the Alzheimer's allele, APOE-e4. The scientists asked the volunteers to fill out detailed questionnaires about their exercise habits during the past 10 years.

Recently, many studies have looked at whether being active can lessen someone's risk for Alzheimer's, but the results have been inconsistent, with some studies, in both animals and people, suggesting that regular exercise has a protective effect and others finding little discernible benefit.

One reason for the inconsistency, Dr. Denise Head, an associate professor of psychology at Washington University who led the study, suspected, might be that many earlier studies did not differentiate between people with the e4 variant and those without, and each group, at least potentially, could respond differently to exercise.

"The good news is that we found that activity levels, which are potentially modifiable, could have an impact" on plaque accumulation - and presumably on the course of Alzheimer's - in people with a genetic predisposition to the condition, Dr. Head says.

But the findings came with a downside, too. An overwhelming majority of the people in the study were sedentary, and for them, an inactive lifestyle seemed to be accelerating the accumulation of amyloid plaques. Those with the e4 variant who rarely or never exercised had the most plaques, putting them at heightened risk for the memory loss of Alzheimer's in the years to come.

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